duplicate
 

how do i duplicatew an object so that when i make changes on nurbs etc, it mirrors the changes for me and does the same on the other side?

When you are about to dublicate (and making sure that your pivot point is at the right place)....go into edit...duplicate options....make one of the scales -1 (it depends what way you want to duplicate...normally it's either -1 in x or z)...and to get it so that when you change one thing on one side and it applies it to the other...you have to select "instance" in the geometry type